Transient absorption measurement, which is also called Flash photolysis, is a method to observe change in absorption of monitor light (called probe light) caused in a sample in a short time scale after irradiation of pulsed light (called pump light) on the sample which induces excitation and photoreaction in the sample material. There are two major methods: one is a CW (continuous-light) probe method which is used in a slower timescale, and the other is a pump-probe method which is used in a pico-and femtosecond time region.
The CW probe method uses continuous wave light or flash lamp light having relatively long light emission time as probe light (see, for example, Patent Document 1). Under the CW probe method, response time or temporal gate width that a detector has, determines a limitation to time resolution. Typically, time resolution of about 10-50 nanoseconds is the limitation in the CW probe method. Since a shorter time region than this region is a so-called high frequency range in a GHz band, there has been a problem that it is difficult to obtain an accurate transient absorption signal free from distortion.
On the other hand, pulse light is used for probe light in the pump-probe method. Specifically, a pulsed light beam with a pulse width of picosecond or less emitted from a single light source is divided into two beams. One of the two beams irradiates the sample as pump light and induces photochemical reaction in the sample. The other one passes through the sample as probe light after being delayed by an optical delay stage. Delay time between the pump light and the probe light is provided in a desired length. Hence, time-resolved transient absorption data can be obtained by scanning the delay time.
Under the pump-probe method, information only at one moment in the rapid reaction, i.e. in time width corresponding to the pulse width, after the pump light irradiation can be captured even by a relatively slow detector because pulse light is used as probe light. The obtained information has time resolution corresponding to pulse width of probe light, and hence, if the pulse width is femtosecond, the time resolution will also be femtosecond. However, the delay time of the probe light pulse from the pump light pulse practically has an upper limit because the delay time is generated by a mechanical delay stage. Specifically, it is difficult to use 1.5 meter or longer delay stages which corresponds to 5 nanoseconds based on speed of light. Accordingly, the pump-probe method has an upper time limit and is not suitable for the measurement in the time region longer than 5 nanoseconds.
The performance that is demanded in a recent transient absorption method is measurement capability in a time region from 100 picosecond to 1 millisecond covering 5 to 50 nanoseconds where either CW probe methods or pump probe methods are not suitable to be used (“gap region” in conventional transient absorption measurements). One of methods that realize those measurements is use of a streak camera in a CW probe method (see, for example, Patent Document 2). However, a streak camera is an exceptionally expensive detector, and also has a problem that one model cannot cover a wide time range. Several methods have been proposed in which delay time up to millisecond is generated actively and electrically by probe light synchronizing with pump light (see, for example, Non-patent Documents 1, 2). However, any of them has a problem that measurement in a wide time-domain requires very long time since only a single probe light pulse is used for a single pump light pulse.